In recent years, virtually all golf balls are of a solid construction, typically including with a solid core encased by a cover, both of which can have multiple layers, such as a dual core having a solid center and an outer core layer, or a multi-layer cover having an inner and outer cover layer. Golf ball cores and/or centers are formed from a thermoset rubber composition with polybutadiene as the base rubber. The cores are usually heated and crosslinked to create a core having certain pre-determined characteristics, such as compression or hardness, which result in a golf ball having the properties for a particular group of players, whether it be professionals, low-handicap players, or mid-to-high handicap golfers. From the perspective of a golf ball manufacturer, it is desirable to have cores exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell golf balls suited to differing levels of ability.
There remains a need, however, for golf ball constructions that allow differing properties to be achieved. One such novel construction with no past commercial success is a golf ball having a hollow core—meaning the innermost portion of the core is hollow surrounded by a ‘shell layer’ and one or more core and cover layers. While, in the past, many commercially-available golf balls have been constructed with non-solid centers, such as liquid centers, very few golf balls having hollow centers have ever been constructed.
While the patent literature references, mostly in a cursory manner, a hollow core as a suitable general alternative construction, very few are actually directed to a hollow core golf ball. For example, U.S. Pat. No. 6,315,683 is generally directed to an over-sized (greater than 1.70 inches) hollow solid golf ball where the hollow core is contained in a thermoset rubber layer and covered with a single ionomer cover. More recently, U.S. Pat. No. 8,262,508 generally describes a golf ball having a hollow center, a mid-layer, an inner cover, and an outer cover. The hollow center and mid-layer are both formed from a thermoset rubber composition, and a conventional ‘positive hardness gradient’ (layer hardness gets softer in the direction of the interior of the layer). The hollow ‘space’ has a diameter of 0.08 to 0.5 inches and the core layer has a low surface hardness of 25 to 55 Shore C. The golf ball is covered by a harder ionomer outer cover and a softer ionomer inner cover.